A photoelectric conversion device for converting light into electricity by using photoelectric effect performed inside a semiconductor has been spotlighted and vigorously developed in recent years. Among the photoelectric conversion devices, a silicon-based thin-film photoelectric conversion device may be expected for lower cost by reason of being formed on a glass substrate or a stainless-steel substrate with large area at low temperature.
The thin-film photoelectric conversion device generally includes a first electrode, one or more semiconductor thin-film photoelectric conversion units, and a second electrode laminated sequentially on a substrate with an insulating surface. The photoelectric conversion unit generally has a structure of a p-type layer, an i-type layer, and an n-type layer stacked in this order; the photoelectric conversion units such that an i-type photoelectric conversion layer, which occupies the main part thereof, is amorphous are referred to as amorphous photoelectric conversion units, and the photoelectric conversion units such that an i-type layer is crystalline are referred to as crystalline photoelectric conversion units.
A photoelectric conversion device having a structure referred to as a multi-junction type, in which two or more photoelectric conversion units are stacked, is known for improving conversion efficiency of a photoelectric conversion device. In a multi-junction type photoelectric conversion device, a photoelectric conversion unit including a photoelectric conversion layer with a large optical band gap is disposed on a light incidence side of the device and one or more photoelectric conversion units including a photoelectric conversion layer with a small band gap are sequentially disposed therebehind, so that photoelectric conversion of incident light over a wide wavelength range may be performed and an improvement in conversion efficiency for the entire device is intended by effectively utilizing incident light.
In the present application, the photoelectric conversion unit disposed relatively on the light incidence side is referred to as a front photoelectric conversion unit, and the photoelectric conversion unit disposed adjacently on the far side from the light incidence side relatively thereto is referred to as a back photoelectric conversion unit. In a case where a photoelectric conversion device has three or more stacked photoelectric conversion units, the back photoelectric conversion units disposed secondly or more from the light incidence side are regarded as front photoelectric conversion unit(s) and the back photoelectric conversion unit disposed adjacently on the far side from the light incidence side relative to the front photoelectric conversion unit(s) exists by plurality.
The above-mentioned multi-junction structure allows incident light to be effectively utilized, however, the properties of the entire multi-junction photoelectric conversion device may be restricted. Particularly, a short-circuit current density of the entire multi-junction photoelectric conversion device is restricted to a short-circuit current density of a photoelectric conversion unit having smaller short-current density. Accordingly, a balance between short-circuit current density generated in each of the photoelectric conversion unit needs to be achieved for improving the properties of the entire multi-junction photoelectric conversion device.
In this point of view, a stacked type photoelectric conversion device having a structure wherein a conductive interlayer having both light transmission and light reflectivity is interposed between laminated photoelectric conversion units has been proposed in recent years. In this case, part of the light reaching the interlayer may be reflected to increase light absorption in a front photoelectric conversion unit located on the light incidence side more than the interlayer and a photocurrent value of the front photoelectric conversion unit is increased.
For example, when an intermediate reflecting layer is inserted into a hybrid photoelectric conversion device formed of an amorphous silicon photoelectric conversion unit and a crystalline silicon photoelectric conversion unit, photocurrent generated in the amorphous silicon photoelectric conversion unit may be increased without increasing the film thickness of an amorphous silicon layer.
In addition, a deterioration of properties of amorphous silicon photoelectric conversion may be repressed. This is because the photodegradation, which becomes remarkable in accordance with an increase in the film thickness of an amorphous silicon layer, may be repressed, since the film thickness of the amorphous silicon layer necessary for obtaining the same current value may be thinned.
It is preferable that such an interlayer selectively reflects a wavelength range of light absorbed in a front photoelectric conversion unit and selectively transmits a wavelength range of light absorbed in a back photoelectric conversion unit. The interlayer is formed of a plurality of thin films having different refractive indexes, so that interference of light may be caused to improve transmittance or reflectance in a specific wavelength range.
When the film thickness of the interlayer is increased, a higher-order interference wave may be utilized and transmittance or reflectance may be changed more steeply with respect to the wavelength. However, the simple thickening of the film increases light absorption and series resistance derived from the components.
As an example in which such a conductive interlayer with both light transmission and light reflectivity is inserted, Patent Document 1, for example, discloses that an interlayer comprising only a conductive oxygenated silicon layer is inserted between photoelectric conversion units to thereby control reflection and transmission amounts of light and increase short-circuit current density as compared with the case where no interlayer is inserted. However, considering that the refractive index of the conductive oxygenated silicon layer is approximately 1.95 and the refractive index of a silicon layer forming the photoelectric conversion units is approximately 3.3, satisfactory reflection properties are not obtained for the reason that a difference of refractive indexes is not sufficient.
Patent Document 2 discloses that a multilayer film in which a plurality of materials are alternately stacked is inserted as an interlayer. However, a polycrystalline silicon layer is used as part of the multilayer film, in this case it is conceived that wavelength-selectivity is improved by refractive index differences in the multilayer film, but still sufficient reflection properties are not obtained by reason of being the same material and refractive index as a silicon layer forming the photoelectric conversion units. The insertion of the polycrystalline silicon layer having an optically significant thickness causes absorption loss in the polycrystalline silicon layer.
In the meantime, a thin-film solar cell as a typical example of a thin-film photoelectric conversion device has been diversified in recent years. A crystalline thin-film solar cell has been developed in addition to a conventional amorphous thin-film solar cell, and a hybrid thin-film solar cell having a stack of these solar cell units has been put to practical use. A thin-film solar cell generally includes a transparent conductive film, at least one or more photoelectric conversion units, a transparent electrode layer, and a high reflecting electrode layer stacked sequentially on a transparent insulating substrate located on the light incidence side. Each photoelectric conversion unit includes an i-type layer sandwiched between a p-type layer and an n-type layer.
An i-type layer as a substantially intrinsic semiconductor layer comprises a large portion of the thickness of the photoelectric conversion unit, and the photoelectric conversion occurs mainly in this i-type layer. Accordingly, the film thickness of the i-type layer as a photoelectric conversion layer is preferably thicker for light absorption, and thickening the i-type layer more than necessary increases cost and time for accumulation thereof. On the other hand, the p-type and n-type conductive layers serve to cause diffusion potential in the photoelectric conversion unit, and the value of this diffusion potential influences the value of open-circuit voltage as one of the important properties of a thin-film solar cell.
However, these conductive layers are inactive layers and have no contribution to photoelectric conversion, and thus light absorbed by impurities doped into the conductive layers does not contribute to electric power generation but becomes loss. Accordingly, the film thickness of the p-type and n-type conductive layers is preferably thinned as much as possible within a range of causing sufficient diffusion potential. With regard to the above-mentioned photoelectric conversion unit, whether the p-type and n-type conductive layers included therein are amorphous or crystalline, the photoelectric conversion unit such that the i-type photoelectric conversion layer is amorphous is referred to as an amorphous photoelectric conversion unit, and the photoelectric conversion unit such that the i-type layer is crystalline is referred to as a crystalline photoelectric conversion unit.
The term ‘crystalline’ in the present application includes a partially amorphous state as generally used in the technical field of a thin-film photoelectric conversion device. One example of a thin-film solar cell having an amorphous photoelectric conversion unit includes an amorphous thin-film silicon solar cell such that amorphous silicon is used for the i-type photoelectric conversion layer. One example of a thin-film solar cell having a crystalline photoelectric conversion unit includes a crystalline thin-film silicon solar cell such that microcrystalline silicon and polycrystalline silicon are used for the i-type photoelectric conversion layer.
Incidentally, examples of a technique for improving conversion efficiency of a thin-film solar cell include stacking two or more semiconductor thin-film photoelectric conversion units into a tandem type structure. In this technique, a photoelectric conversion unit having a photoelectric conversion layer with large band gap is disposed on the light incidence side of a thin-film solar cell and a photoelectric conversion unit having a photoelectric conversion layer with a small band gap is sequentially disposed therebehind, so that photoelectric conversion of incident light over a wide wavelength range may be performed and thereby an improvement in conversion efficiency for the entire thin-film solar cell is intended.
Among such tandem type thin-film solar cells, a thin-film solar cell including both an amorphous photoelectric conversion unit and a crystalline photoelectric conversion unit may be referred to particularly as a hybrid thin-film solar cell.
For example, in a hybrid thin-film solar cell in which an amorphous silicon photoelectric conversion unit such that an i-type amorphous silicon with a wide band gap is used for a photoelectric conversion layer and a crystalline silicon photoelectric conversion unit such that an i-type crystalline silicon with a narrow band gap is used for a photoelectric conversion layer are stacked, a wavelength of light capable of being subjected to photoelectric conversion by the i-type amorphous silicon is up to approximately 800 nm on the long-wavelength side and meanwhile light with a wavelength of up to approximately 1100 nm longer than it may be subjected to photoelectric conversion by the i-type crystalline silicon. Therefore, incident light in a wider range may be effectively subjected to photoelectric conversion in a hybrid thin-film solar cell
In order to utilize incident light on a photoelectric conversion unit more effectively, a high reflecting electrode layer made of metallic materials with high light reflectance is formed in a thin-film photoelectric conversion device. Transmitted light without being absorbed in a photoelectric conversion unit is reflected by the high reflecting electrode layer and enters the photoelectric conversion unit again to be subjected to photoelectric conversion, so that conversion efficiency of a thin-film photoelectric conversion device is improved.
On the other hand, a transparent electrode layer is provided between the photoelectric conversion unit and the high reflecting electrode layer to intend an improvement in adhesion properties between the photoelectric conversion unit and the high reflecting electrode layer and prevent metallic materials for the high reflecting electrode layer from diffusing and mixing into the photoelectric conversion unit. However, the insertion of the transparent electrode layer is effective in the above-mentioned intention, but yet greatly influences the film quality of the high reflecting electrode layer depending on the forming conditions of the transparent electrode to cause functional depression as the reflecting layer.
Further, the transparent electrode layer occasionally becomes a barrier in electrical contact between the photoelectric conversion unit and the high reflecting electrode layer and does not sufficiently function as a take-out electrode, so that the properties as a solar cell are occasionally deteriorated.
In addition, absorption loss is caused in an interface between the transparent electrode layer and the high reflecting electrode layer to substantially decrease incident light into a silicon layer and decrease the properties of a solar cell.
With regard to this problem, in Patent Document 3, an element for controlling electrical conductivity is contained in the transparent electrode layer to change this element in a film thickness direction, so that electrical conductivity is changed to intend an improvement in electrical contact. However, in this method, series resistance of a solar cell can be decreased but yet in the case of increasing the content of the element for decreasing electrical conductivity, transmittance is decreased and incident light into a silicon layer is decreased.
Patent Document 3 is silent to solutions for interface absorption between the transparent electrode layer and the high reflecting electrode layer. Patent Document 4 discloses that interface absorption loss is decreased by inserting a refractive index control layer between the transparent electrode layer and the high reflecting electrode layer. However, in Patent Document 4, the refractive index control layer is not made of conductive materials and only an insulator is disclosed, the refractive index control layer act as a barrier in electrical contact between the transparent electrode layer and the high reflecting electrode layer. The point that the properties are greatly deteriorated due to an increase in series resistance is not considered at all.    Patent Document 1: Japanese Unexamined Patent Publication No. 2005-135987    Patent Document 2: Japanese Unexamined Patent Publication No. 2001-308354    Patent Document 3: Japanese Unexamined Patent Publication No. 5-110125    Patent Document 4: Japanese Unexamined Patent Publication No. 2006-120737